MINOR: qpack: properly handle invalid dynamic table references

MINOR: qpack: properly handle invalid dynamic table references

Return QPACK_DECOMPRESSION_FAILED error code when dealing with dynamic
table references. This is justified as for now haproxy does not
implement dynamic table support and advertizes a zero-sized table.

The H3 calling function will thus reuse this code in a CONNECTION_CLOSE
frame, in conformance with the QPACK RFC.

This proper error management allows to remove obsolete ABORT_NOW guards.